This invention is in the field of joining pressure pipe, more specifically, those with a bell on one end and the other end plain.
Cast iron pressure pipe, with a bell on one end and the other plain or beveled, have for many years been joined utilizing a rubber gasket which is compressed between the walls of the bell and the outside wall of the plain or beveled end. The most successful of such systems provide an elongated groove in the bell with a gasket sealing wall and a gasket restraining portion as well as lip and wall portions which guide and limit travel of the plain end as it passes through the bell opening and the rubber gasket. Such a pipe joint is described in U.S. Pat. No. 2,953,398 issued Sept. 20, 1960. The rubber gaskets used as a part of such systems have a retainer part and a sealing part. While there may be added lip seals or various additions to the gasket, the essential portions are merely a portion which will cause the gasket to remain in its proper position during joint assembly and use and a portion which will maintain sealing engagement with the two pipe sections at all times. Small diameter cast iron pipe are readily sealed by such a system since their cross section deviates only slightly from a true circle and since they are cast to relatively close tolerances. When larger pipe are produced and the pipe wall thickness is increased only a small amount, several problems of major proportion become apparent. These problems include the occurrence of oval cross sections and the need for greater casting tolerances. Many solutions to these problems have been devised with varying degrees of success due mainly to the economics of the solution. In general, the gasket utilized in such joining systems must fill the space between the bell inside sealing wall and the outside wall of the plain end and remain compressed to some degree regardless of ovalness or tolerance. In the design of such systems when sizing the cross section of the gasket and determining its maximum and minimum tolerances to be allowed, two extreme conditions must be considered. One condition is when the gasket is under minimum compression, e.g., when the space to be filled for the maximum size bell and the minimum size plain end, the plain end resting in the invert of the bell and the ovalness of the two sections oriented so as to place the maximum gap at the upper extremity. The other condition is when the gasket is under maximum compression, e.g., when the bell cavity is minimum size and the plain end maximum size. The first condition is usually referred to as a loose joint and the second as a tight joint. If in pipe larger than about 24 inches in diameter, a bell and a gasket of reasonable size are provided such that the gasket will fill the gap occurring in the loose joint and remain in radial compression, then when a tight joint condition is encountered the assembly force required is quite unreasonable. Conversely, if a small enough gasket is provided to keep the assembly force within reasonable limits, the gap occurring between the two sections of a loose joint will not be filled and a leak will occur. Solutions to this problem include reducing the tolerances to such an extent that machining or grinding the joining surfaces is necessary to achieve the tolerances, marking the major axis and laying each pipe with the major axis of the plain end in a vertical position, providing jacks to force the pipe into a circular cross section when joining, perforating a portion of the sealing bulb on the gasket to allow it to compress more readily and various additions to the gasket, such as lip seals. All of these solutions add substantially to the cost of the product. It should be noted that when relatively flexible pipe, such as ANSI A 21.51 Class 50 thirty-six inch diameter ductile iron pipe, are being joined, the "loose joint" condition will allow a gap which is much greater than that which would be expected from the two oval sections being joined as described above. Field tests have revealed that under certain external loading the circumference of the plain end tends to conform to the inside circumference of the bell lip for a large portion thereof, creating an extremely large gap at the upper extremity.